Continuous tone development method for xerographic printing



CONTINUOUS TONE DEVELOPMENT METHOD FOR XEROGRAPHIC PRINTING FIG. 2

I0 I IITI 7 FIG. 4

1 FIG. 6

W I8 I I I I I2 7 II J I I I I I I I I I I M- W. DOWLEY Filed March 20,1964 I 0% III III I I I III III 9 m fl++++++++++++++++ \III/ IIII III/II /III II IrIIr\ IIIIIIIIIIIIIIIIII IIII III A rii 23, 1968 FIG I FIG.3

FIG. 5

FIG. 7

7 FIG. 8

INVENTOR. MARK W. DOWLEY BY Z 9 ATTORNEY United States Patent 3,379,553CONTINUOUS TONE DEVELOPMENT METHOD FOR XEROGRAPHIC PRINTING Mark W.Dowle-y, San Jose, Calif., assignor to International Business MachinesCorporation, New York, N.Y., a corporation of New York Filed Mar. 20,1964, Ser. No. 353,440 1 Claim. (Cl. 11717.5)

ABSTRACT OF THE DISCLOSURE A method of continuous tone xerographicdevelopment wherein the phenomenon that toner attraction is governed byelectrical field distribution is recognized and utilized such that theelectrical field distribution of an insulating member is made to beessentially that of a free body in space by applying a very highpotential to the conductive backing plate of the same polarity as thecharge on the insulating surface.

This invention relates to xerography and electrophotography generallyand more particularly to a method of continuous tone development.

In xerography, plates having a photoconductive insulating materialoverlying a conductive backing member are sensitized by placing auniform electrostatic charge on the surface of the photoconductiveinsulating layer while the plate is kept in darkness. A latent image isthen formed on the surface of the insulating layer by exposing it to alight pattern. Exposure to light causes areas of the photoconductiveinsulating layer to become conductive and charges on the surface arethus dissipated. Areas not exposed to light continue to act asinsulators and, therefore, retain their electrostatic charges. Theelectrostatic charge pattern formed may be developed by bringingelectrostatically charged particles into contact with the surface areacarrying the charge pattern or the pattern may be otherwise utilized. Adeveloped image composed of particles deposited in conformity with theelectrostatic charge pattern may be transferred from the surface of theplate to sheets or webs through use of electrostatics or other methodsknown to the art and may be there permanently affixed.

Continuous tone copies without distortion of the original are in almostall instances desired. Continuous tone development in xerography is,however, rarely achieved due to a phenomenon which occurs in the casewhere an insulating member carrying a charge is backed by a conductor asin the usual xerographic situation. Upon developing a xerographic plate,where a large area of continuous toning should occur, unless laterdiscussed precautions are taken, emphasized development takes placealong the edges of the large area and little or no toner is attracted tothe center portions. To prevent such development, development electrodesare normally used. A development electrode is positioned at a slightdistance from the photoconductive insulating layer during development.The development electrode is composed of a conductive material and iseither biased or maintained at the potential on the conductive backingmember. This will cause an increase in the lines of force extendingoutwardly from the plate member creating electrostatic fields which,when developed, will produce continuous tone copy.

Several problems are, however, attendant to the use of a developmentelectrode. For instance, the development electrode, since it is spacedat a slight distance from the sensitive layer, tends to build up on itssurface a coating of developer material. While this coating remainsthin, the image which is developed is generally not affected; however, acoating of developer material on the develop- 3,379,553 Patented Apr.23, 1968 ment electrode interposes a new surface between the sensitivelayer and the development electrode. If the particles carryelectrostatic charges, this coating may effectively change the biaspotential on the electrode, which substantially decreases the beneficialaspects of the development electrode during development. This coatingalso adds additional and uncontrollable problems. The lack of controlfollowing heavy disposition is to some extent attributed to the factthat the coating which forms is generally uneven and follows unknown anddifferent patterns. A heavy coating may also drop agglomerates ofdeveloper powder to the plate surface resulting in a non-uniformdistorted development of the electrostatic image pattern. Moreover, sucha coating may cause irregular and uncontrolled flow patterns of thedeveloper material being presented to the plate surface which will causestreaking of the final developed image.

These problems are well known in the art. Several systems have beendeveloped in an attempt to overcome the problems attendant to the use ofdevelopment electrodes. Thus, in a patent to Crumrine et al., 2,784,694,a segmented development electrode is provided, the segments of which arerotatable for brushing by an associated brush to effect cleaning. Thispatent also states that another apparatus developed for preventing thisbuildup of toner on a development electrode was one wherein thedevelopment electrode was caused to constantly move back and forthtransversely across the xerographic plate. Brushes were provided toengage the surface of the electrode when removed from its operativeposition relative to the xerographic plate for cleaning the powder fromthe surface. Another apparatus provided is shown in a patent to Hayford,2,844,123, wherein a continuous belt develop-ment electrode is providedwhich rotates past the sensitive surface and then by a vacuumcleaner-brush arrangement whereby toner is cleaned from the developmentelectrode. Another apparatus more recently devised is that covered in apatent to Clark et al., 2,952,241. The apparatus of this patent isdesigned to overcome the problem of a rigid electrode which would notallow toner to pass between the electrode and the sensitive surface inthe case of toner buildup. Thus, the development electrode is madeflexible to permit toner particles to pass between it and the imagebearing surface thereby avoiding undesirable pileups of toner.

It can be seen, therefore, that the prior art systems have accepted theproblems attendant the use of development electrodes and attempted tolive with them by means of the various systems above described.

It is and object of the present invention to provide a novel method ofxerographic development.

Another object of the present invention is to provide a new method ofxerographic development wherein continuous tone copies are provided.

Another object of the present invention is to provide a new method ofxerographic continuous tone development which does not necessitate adevelopment electrode.

Another object of the present invention is to provide a new method ofcontinuous tone development which provides results far superior to thoseobtained through use of conventional development electrodes.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which:

FIG. 1 is a view which aids in understanding the phenomenon on which thesubject method is based;

FIG. 2 is a view representing the electric field distribution in andaround 'a uniformly charged insulator backed by a grounded conductiveplate;

FIG. 3 is a view illustrative of the charge pattern in and around arandomly charged insulator backed by a grounded conductive plate;

FIG. 4 is a graphical representation of the electric field distributionin and around the insulator and plate combination of FIG. 3;

FIG. 5 is a view illustrative of the toner deposition which would occurif the plate and insulator combination of FIG. 3 were toned;

FIG. 6 is a view representative of the electric field distribution of 'adielectric sheet charged in conformity with the charge pattern of FIG. 3and wherein the insulating sheet is isolated in space;

FIG. 7 is a view illustrative of one method of separating a flexibleinsulating member from a rigid conductive base member; and

FIG. 8 is a view illustrative of 'a method of simulating isolation of amember in space to provide electric field distribution normal to itsface.

Briefly, in accordance with the subject invention, a method ofcontinuous tone xerographic devclopment is presented wherein thephenomenon that toner attraction is governed by electric fielddistribution is recognized and utilized such that the electric fielddistribution of an insulating member is made to be essentially that of afree body in space either by separation of the charged insulating memberfrom the conductive backing plate or by applying a very high potentialto the conductive backing plate of the same polarity as the charge onthe insulating surface.

One view expressed with respect to what causes the edge effect (i.e.,the emphasized deposition of toner at the edge of a large charged areaand not in the center) is that this behavior is due to the greater fieldgradient, or charge density at the edges of the electrostatic image,which are not present at the center of a large uniformly charged area.It is submitted that a more accurate view is that electrically chargedbodies experience a force when in an electric field and in xerographycharged toner particles are attracted to and are retained by regions ofthe xerographic plate which have a sufiiciently strong component of theelectric field of the correct polarity perpendicular to and above theplate.

Furthermore, the density of toner adhering to a region of the plate willbe proportional to the vertical component of electric field in thatregion, i.e., the greater the electric field, the greater the density oftoner deposited. For the same reasons, in the cascade developmenttechnique, the deposition of toner on the xerographic plate will dependon whether the toner particle experiences a force, due to the electricfield of the electrostatic image, sufficient to overcome the attractiveforce of the carrier. In other words, toner will be deposited if theelectric field above the image is greater than that of the carrier.Tests of an electrostatic image before application of toner to determineimage quality should be concerned with a measurement of the electricfield at the surface of the plate and not with the voltage or chargedensity which may or may not be relevant to the development process.Electric fields strength, however, is always relevant.

The truth of the above may be proven by recourse to fundamentaldefinitions and concepts of electrostatics. According to Coulombs Law, acharge q experiences a force F when in the vicinity of another chargeQ2. The force F is given by the equation F=q q /41ree r (1) wherein F isin newtons if (1 and :1 are in coulombs, r the separation between thecharges is in meters, 6 is the dielectric constant of the medium and cthe permitivity of free space equal to (1/3671') 10- farad/meter.

The electric field E at the position of q due to :1 is defined by theequation traction between carrier and toner particles originates in themutual transfer of electric charge which occurs when the carrier andtoner, both neutral originally, are brought into contact. Thisphenomenon, triboelectrification, is 'a result of the greater electronafiinity of one material over the other. The attractive force could berepresented by Eq. 1 with g and r the separation of the charges.Experimentally, it is found that a certain electric field strength isrequired to remove the toner from the carrier.

When a charged conducting sphere 1 is dipped into a dish 2 of carrierplus toner 3, as illustrated in FIG. 1, it

is found that the density of toner 4 adhering to the sphere isproportional to the field strength at the surface of the sphere v/rwhere v is the voltage of the charging battery 5 and r the radius of thesphere. For spheres of different r'adii, equal densities are found forequal field strengths not for equal voltages. Roughly, the toner densityp on a conducting sphere after dipping is given by an expression of theform where E is the field strength at the surface of the sphere, E isthe threshold field below which no toner adheres to the sphere and c isa proportionality constant.

It should be clear from the above that whether toner :0 will beattracted to and adhere to a xerographic plate depends on whether thereis a sufficiently strong component of electric field strengthperpendicular to the surface of the plate. More particularly, thedensity distribution of toner will depend directly on the distributionof electric field strength over the surface of the plate. As shown inFIG. 2, a uniformly charged dielectric layer 6 is backed by a groundedconductive plate 7 appears, in so far as its electric field distribution8 and 9 is concerned, very much like a parallel plate condenser, i.e.,the field 9 is strong between the plates and is essentially zero outsidethe plates except at the edges where fringing fields 8 protrude beyondthe plates. With this field distribution, it is expected and confirmedthat no toner is deposited on the surface when development is attemptedother than at the edges. A photoconductive plate 10, however, asillustrated in FIG. 3, which, having been uniformly charged and thenexposed in some regions to light, will have charged areas 11 and 12 anduncharged area 13 corresponding respectively to those areas which werenot ex- 30 posed and those which were. The electric field distributionof the charged areas 11 and 12 will again correspond approximately tothat of a similar sized parallel plate capacitor while the unchargedarea 13 will have no electric field. FIG. 4 illustrates the verticalfield intensity ;5 (E) at the surface of the plate-photoconductorcombination of FIG. 3 while FIG. 5 illustrates the toner density 7)after development. It will be noted that large uniformly charged areas11 develop only at the edges 15 and 16 (the edge effect) due to the factthat only at the edges 14 and 15 is there appreciable electric fieldstrength.

on development. This sharp edge phenomenon is consistent with thepostulate that the toner is attracted in proportion to the electricfield strength and not in proportion to voltage contrast or fieldgradient.

From these considerations the subject method of obtaining fieldintensity near the surface of the photoconductive material which isproportional to the charge density and which results in uniform orcontinuous tone development of charged areas evolved. As shown in FIG.7, the photoconductive member 1-8 is peeled from the backing plate 19and removed therefrom. In this isolated condition, as illustrated inFIG. 6, the electric field distribution, as illustrated by the arrows20, of the charge on the photoconductive surface is radically differentfrom that of the combined photoconductive layer It) and conductive plate7a illustrated in FIG. 3. The electric field distribution of an isolatedcharge area is such that the electric field strength outside the surfaceis proportional to the charge density on the surface, and thus, uniformtoner deposition on a uniformly charged surface can be obtained therebyeliminating the edge effect. Note the illustrated electric fielddistribution where the photoconductive layer 18 has been peeled from thebacking plate 19.

Application of a potential to the conductive plate in such a Way as torepel the photoconductive layer will aid in overcoming the electrostaticattractive force to facilitate separation.

Another method of simulating or providing the effect of a chargedinsulator arranged in space to accomplish results nearly as good as thepeeling technique is that illustrated in FIG. 8 wherein the conductivebase plate 21, which supports the photoconductive surface 22 is raisedto a high electric potential of the same sign as the charge originallydeposited on the photoconductive plate. This has the effect of changingthe electric field intensity pattern at the surface from that of aparallel plate condenser, as illustrated in FIG. 3, to that of twoadjacent layers of similar charge, as illustrated in FIG. 7. The resultis that the electric field intensity in the region directly above acharged area is substantially increased and, hence, toner will bedeposited there producing a uniform toner development or developmentproportional to the charge density.

'In the above described manner, there has been provided a novel methodwhich is conductive to continuous tone development in xerographicprinting. This method requires neither the conventional developmentelectrode used in the prior art to obtain continuous tone developmentnor does it consequently require any of the peripheral means utilized inconjunction with the development electrode to assure proper functioning.Instead, realization of the phenomenon that toner attraction is due notto field gradient, but due to the electric field distribution along withtwo methods of providing electric field distribution normal to thesurface of the member to be toned, is considered to be, in light of thestate of the art, a new and novel, useful method of continuous tonedevelopment in xerographic printing.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in the form and detailsmay be made therein without departing from the spirit and scope of theinvention.

What is claimed is:

1. A method of continuous tone printing comprising a the steps of:

forming a latent electrostatic charge pattern on one side of aninsulating member while the other side thereof is backed by a conductivemember,

applying a potential of suificient magnitude of like sign to that ofsaid electrostatic charge pattern solely to said conductive backingmember to cause the electric field distribution attendant theelectrostatic charges to include components substantially perpendicularto and rising above said one side of said insulating member, theelectric field intensity in the region directly above said chargedpattern being thus substantially increased due solely to said appliedpotential,

cascading a mixture of solid toner and carrier particles to saidelectrostatic charge pattern while said potential is applied, said tonerparticles being of opposite sign to that of said electrostatic chargepattern,

whereby said toner particles adhere to said charge pattern forming avisible image of continuous tone quality.

References Cited UNITED STATES PATENTS WILLIAM D. MARTIN, PrimaryExaminer.

0 E. J. CABIC, Assistant Examiner.

